In continuous polymerization processes, especially those of the mass type, it is common to conduct a polymerization reaction in a reactor until a predetermined level of conversion is reached after which a fluid mass comprising a mixture of unconverted monomer and polymer is fed to a separator where, by physical means, monomer is separated from polymer. Monomer is then optionally and preferably recycled while polymer product is then removed from the system. A common example is the continuous mass polymerization of styrene to polystyrene followed by devolatilization of the heated viscous mixture to polymer and monomer to separate out unreacted styrene for recycle.
In such a continuous process in which many control schemes may be used it is common to regulate the level of reactants in the reactor, and to charge monomer to, or remove a product mixture from, the reactor or both, at respective rates which maintain the reactor at a predetermined fill level, the reactor pressure and temperature being controlled at predetermined respective values so as to achieve a desired level of conversion of monomer to polymer. To regulate fill level in a reactor, one may conventionally employ a level control assembly which directly controls fluid fill level in the reactor or one may conventionally employ weigh cells which weigh the quantity of materials in the reactor and indirectly control fluid fill level therein. Appropriate signal generating and feedback loops then function to regulate the quantities of monomer fed to, or reaction product removed from, the reactor.
Such conventional continuous process controls work well with conventional reactors, such as those of the vertically stirred kettle or plug-flow type. Recently, however, a different type of reactor has come into use which is of the horizontally continuously stirred tank type, and wherein a mixer assembly therein during operation functions periodically (with each revolution) to sweep out substantially the entire interior volume of the reactor making level sensors difficult to place. This latest reactor, in addition, has a capacity to be operated at variable fillage levels, and, furthermore, the churning action of the mixer assembly therein tends to make the fill level, and reactor gross weight measurement, variable depending on such variables as the quantity of reactants in the reactor instantaneously and the mixer rotational speed. When such a reactor is large relative to the quantity of reactants therein, it can be difficult to accurately weigh differentially and incrementally the quantity of monomer added thereto or of polymer removed therefrom. For these and other reasons those skilled in the art will appreciate, it can be exceedingly difficult to control reactor throughput rates, fill levels, and percent conversion when using such a type of reactor.
There has now been discovered, however, a control system for use in continuous polymerization processes involving one or more zones of polymerization followed by one or more zones of separation which does not involve the use of reactor level control means or of reactor weigh cells and which is especially well adapted for use with such a variable capacity horizontally continuously stirred tank reactor as just described. This control system is simple to operate and economical to construct. It not only controls the relative quantity of polymer in reactor effluent fed to separator but also controls reactor fill level. Monomer feed rate to reactor is fixed at a predetermined value, as are the reactor pressure and temperature. Only reactor effluent comprising a mixture of polymer and monomer is varied. This control system thus both overcomes the disadvantages of prior art control systems and makes possible new and highly convenient and economical continuous polymerization procedures.